Since the 1800's fingerprint information has been collected from human digits and hands by means of ink and paper. For the purposes of this document, the term “fingerprint” is used to refer to the skin surface friction ridge detail of a single digit, or part of the friction ridge detail of a digit, or any portion of the skin surface friction ridge up to and including the entire hand. In recent years, various electronic fingerprint scanning systems have been developed utilizing optical, capacitance, direct pressure, thermal and longitudinal-wave methods. Methods based on longitudinal waves, including ultrasound, have proven to be highly accurate, since they are virtually unaffected by the presence of grease, dirt, paint, ink and other substances commonly found on a person's skin.
Use of ultrasound typically employs a piezoelectric transducer that sends an ultrasonic energy wave (often referred to as a “pulse”) through a transmitting media. The pulse partially reflects back at each media interface. By knowing the speed of the longitudinal wave, and the time at which an ultrasonic energy pulse was emitted, the distance traveled by the pulse can be determined once the reflected portion of the pulse is detected, and this can be done for each reflecting material interface.
Many of the reflected pulses are not of interest. For example, when a fingerprint is of interest, the pulses reflected by interfaces other than where the digit resides are not of interest. Since pulses reflected by the various interfaces will arrive at different times, it is possible to identify those pulses that are of interest by monitoring a time interval during which the reflected pulse for that interface is expected to arrive. This process is often referred to as “range gating” or “biasing”. The reflected pulse received during the expected time is then processed, often by converting it to digital values that represent the signal strength. By graphically displaying this information, a three-dimensional contour map of the object (e.g. a human finger, thumb or other skin surface) that reflected the pulse can be created. With respect to interface surfaces that are not flat, the depth of any gap structure detail (e.g. fingerprint valleys) can be displayed as a gray-scale bitmap image.
Although ultrasound imaging of a fingerprint is superior in detail to a similar image collected by an optical system or other means, the time required to collect a raster scanned acoustic image using a single pixel sweep scanning device is longer than the time needed to collect an optical image of the same size. In such a scanning system, the scanning involves collecting each pixel of image information individually and separately by means of a two-axis mechanical scanning apparatus. There is a need for a device that is faster, but provides the superior detail of an ultrasound system.
Further, there are a number of fingerprint readers employing various techniques of capturing information about a friction ridge for purposes of creating an image of the friction ridge. An inexpensive reader that enjoys utility in many applications, especially small computers, is the swipe sensor. In using a fingerprint swipe sensor, the user moves his digit over a line of sensing elements. For proper operation, the direction in which the digit moves is not parallel to the line of sensing elements. By repeatedly capturing linear images of the digit while the digit is moved over the line of sensing elements, and then combining the linear images, a fingerprint image may be formed. Typically, information is needed about the speed of motion of the digit being swiped in order to properly place the images obtained by the line of sensing elements.